1. Field of the Invention
The invention is directed to methods for the transformation of local measured values of a characteristic of a cornea, particularly an optical characteristic of a cornea, or values derived therefrom in a false color image according to an assignment rule for unambiguous mapping of a first range of values, or range of values, to a set of false colors and apparatus for implementing a method of this kind, particularly apparatus for measuring the topography of a cornea, having a light source for structured illumination, a camera, a display, and a control unit which is connected to the camera and to the display and which is designed to record an image of the cornea by means of the camera for determining local measured values of a characteristic of the cornea, particularly an optical characteristic of the cornea, or values derived therefrom based on the image and for displaying the model on the display in a false color scale according to an assignment rule for the unambiguous mapping of a first range of values to a set of false colors.
2. Description of Related Art
A high-resolution profile of the cornea which determines particularly its imaging characteristics or surface characteristics can be achieved by measuring the topography of the cornea of the human eye (photokeratoscopy or videokeratography), which is known per se. The corneal profile can be used for diagnosing, planning surgical procedures on the cornea, and for adjusting eyeglasses or contact lenses. Typically, a Placido ring projector which projects concentric rings on the cornea is used to determine corneal topography. By imaging reflections of the rings, an optical characteristic of the reflecting surface (i.e., of the cornea) can be determined based on the resulting deformation in the reflected image. For example, heights or local refractive powers (or the corresponding radii of curvature) of the cornea can be measured in this way. Methods for determining corneal topography based on Placido ring projection are described, for example, in U.S. Pat. Nos. 6,213,605, 6,382,794, or 6,257,723.
In the prior art, the measured values recorded at different locations of the cornea are graphically displayed as plane projections in false colors (color coding), which is known as a topography map. In so doing, a fixed range of values is divided into steps (subintervals), each of which is displayed unambiguously by an element of a given set of false colors, for example, color interval(s) or discrete colors. By varying a color component, for example, the hue, saturation or intensity, intermediate steps can be generated. Generally, a standardized assignment rule is used for color coding, for example, based on ANSI Z80.23-2008 or ISO 19980:2005. Images of different patients or of the same patient at different points in time can be compared to one another visually using a standardized color coding which distributes the possible hues over the refractive powers (or radii of curvature) usually occurring at the human eye.
However, owing to the limited quantity of available hues and the limited capability of the human eye to resolve degrees of brightness, this kind of graphic display has the disadvantage that, depending on the range of values, either only local variations of low amplitude around an identical, or at least similar, base value or only an overview of the total topography in which local variations of low amplitude are suppressed can be perceived. Further, it is impossible to show local variations of low amplitude around two (or more) distinctly different base values in a visually perceptible manner. It is known to determine a deviant assignment rule manually in order to observe a determined range of values in a manner specific to the eye, for example; however, this requires expert knowledge owing to the complexity of the possible variants and is time-consuming even for experts.